.TH pgm 5 "12 November 1991"
.SH NAME
pgm - portable graymap file format
.SH DESCRIPTION
The portable graymap format is a lowest common denominator grayscale
file format.
.IX "PGM file format"
The definition is as follows:
.IP - 2
A "magic number" for identifying the file type.
A pgm file's magic number is the two characters "P2".
.IX "magic numbers"
.IP - 2
Whitespace (blanks, TABs, CRs, LFs).
.IP - 2
A width, formatted as ASCII characters in decimal.
.IP - 2
Whitespace.
.IP - 2
A height, again in ASCII decimal.
.IP - 2
Whitespace.
.IP - 2
The maximum gray value, again in ASCII decimal.
.IP - 2
Whitespace.
.IP - 2
Width * height gray values, each in ASCII decimal, between 0 and the specified
maximum value, separated by whitespace, starting at the top-left
corner of the graymap, proceeding in normal English reading order.
A value of 0 means black, and the maximum value means white.
.IP - 2
Characters from a "#" to the next end-of-line are ignored (comments).
.IP - 2
No line should be longer than 70 characters.
.PP
Here is an example of a small graymap in this format:
.nf
P2
# feep.pgm
24 7
15
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
0  3  3  3  3  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0 15  0
0  3  3  3  0  0  0  7  7  7  0  0  0 11 11 11  0  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0  0  0
0  3  0  0  0  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15  0  0  0  0
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
.fi
.PP
Programs that read this format should be as lenient as possible,
accepting anything that looks remotely like a graymap.
.PP
There is also a variant on the format, available
by setting the RAWBITS option at compile time.  This variant is
different in the following ways:
.IX RAWBITS
.IP - 2
The "magic number" is "P5" instead of "P2".
.IP - 2
The gray values are stored as plain bytes, instead of ASCII decimal.
.IP - 2
No whitespace is allowed in the grays section, and only a single character
of whitespace (typically a newline) is allowed after the maxval.
.IP - 2
The files are smaller and many times faster to read and write.
.PP
Note that this raw format can only be used for maxvals less than
or equal to 255.
If you use the
.I pgm
library and try to write a file with a larger maxval,
it will automatically fall back on the slower but more general plain
format.
.SH "SEE ALSO"
fitstopgm(1), fstopgm(1), hipstopgm(1), lispmtopgm(1), psidtopgm(1),
rawtopgm(1),
pgmbentley(1), pgmcrater(1), pgmedge(1), pgmenhance(1), pgmhist(1), pgmnorm(1),
pgmoil(1), pgmramp(1), pgmtexture(1),
pgmtofits(1), pgmtofs(1), pgmtolispm(1), pgmtopbm(1),
pnm(5), pbm(5), ppm(5)
.SH AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
.\" Permission to use, copy, modify, and distribute this software and its
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.\" that the above copyright notice appear in all copies and that both that
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.\" documentation.  This software is provided "as is" without express or
.\" implied warranty.
